Carbon based electrodes

ABSTRACT

A carbon based electrode for the electrochemical reduction of sulfur or oxygen, which comprises an electrode core and, in electrical contact therewith, a structure comprising a porous particulate activated carbon bonded with a polymeric binder material, characterised in that the structure is at least 1 mm thick, in that the particulate activated carbon is prepared from a lignocellulosic material and has the following properties: i) a particle size in the range of from 200 to 850 μm; ii) a pore volume of from 0.45 to 1.0 cm 3  per gram; iii) a surface area in the range of from 800 to 1500 m 2 /g; and in that the binder is used in an amount not exceeding 25% by weight based upon the mixture of activated carbon and binder material.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to carbon, based electrodes and, inparticular, to carbon based electrodes for the electrochemical reductionof sulfur or oxygen.

(2) Description of Related Art

It is well known in the electrochemical field that carbon is not anelectrocatalyst for the reduction of sulfur (Allen, P. L. and Hickling,A., Trans. Faraday Soc., 53 (1957) 1626). Various workers haveendeavoured to find good elecrocatalysts for the reduction of sulfurwhich enable the electrode to operate at a current density of greaterthan 20 mAcm⁻² at an over potential of less than 50 mV. To date the onlymaterials which have been found to be sufficiently active are metalsulfides. However, the materials with the highest activity do not showgood long term stability and are expensive to produce.

A separate problem occurs when carrying out the sulfide/polysulfideredox reduction which is described, for example, in U.S. Pat. No.4,485,154. The current density at an electrode carrying out thesulfide/polysulfide redox reaction is limited by the combined effects ofthe reactions in solution and slow electrochemical reaction kinetics.Many authors (Lessner, P. M., McLaron, F. R., Winnick, J. and Cairns,E.J., J. Appl. Electrochem., 22 (1996) 927-934, Idem., ibid. 133 (1986)2517) have proposed a metal or metal sulfide deposited on a high surfacearea electrode (e.g. an expanded metal mesh) to overcome these effectsby providing a high interfacial area per unit volume and anelectrocatalyst surface. The catalytic electrode surface layers of Ni,Co or Mo metals, or the sulfides of these metals, achieve only a modestcurrent density of 10 to 20 mAcm⁻² at an over voltage of 50 mV.

Electrodes which are surfaced with carbon are recognized to be two tothree orders of magnitude worse than the catalytic electrode surfacelayers of Ni, Co or Mo metals, or the sulfides of these metals.Accordingly, a 50 mV overvoltage would be achieved only at currentdensities of 0.1 to 1 mAcm⁻². For example, overvoltagaes of 300 to 500mV are encountered at 40 mAcm⁻² even when the electrode surface iscoated with a high surface area carbon (see U.S. Pat. No. 4,069,371 andU.S. Pat. No. 4,177,204).

It would be advantageous to be able to use carbon based electrodes forthe electrochemical reduction of sulfur in the sulfide/polysulfide redoxenergy storage system because a carbon based electrode would not sufferfrom degradation due to inter-conversion between various sulfide phases.We have now developed a technique for making carbon based electrodeswhich are electrocatalytically active for the sulfur reduction process.Such electrodes are also suitable for the electrochemical reduction ofoxygen.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a carbon based electrode forthe electrochemical reduction of sulfur or oxygen, which comprise anelectrode core and, in electrical contact therewith, a structurecomprising a porous particulate activated carbon bonded with a polymericbinder material, characterised in that the structure is at least 1 mmthick, in that the particulate activated carbon is prepared from alignocellulosic material and has the following properties:

i) a particle size in the range of from 200 to 850 μm;

ii) a pore volume of from 0.45 to 1.0 cm³ per gram;

iii) a surface area in the range of from 800 to 1500 m²/g;

and in that the binder is used in an amount not exceeding 25% by weightbased upon the mixture of activated carbon and binder material.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of the process for the productionof a carbon based electrode.

DETAILED DESCRIPTION OF THE INVENTION

The activated carbon which is used in the present invention is preparedfrom a lignocellulosic material such as nut shells or fruit stones. Theactivated carbon may be prepared by oxidising nut shells or fruit stonesby contact with an oxidising acid at an elevated temperature to producean intermediate product which is then carbonised to give the activatedcarbon according to the teaching of EP-A-0395353. Another method ofpreparing the activated carbon used in the present invention is by thesteam activation of a carbonised product prepared from a lignocellulosicmaterial. The activation method is known in the art and is generallycarried out at a temperature of from 600 to 1000° C. A further method ofactivating carbon, which is well known in the art, is the air activationmethod which also produces an activated carbon which can be used in thepresent invention.

The activated carbon product from the treatment of the lignocellulosicmaterial obtained is then passed through a series of sieves in order toisolate the particle fraction having a particle size in the range offrom 200 to 850 μm for use in the present invention. It is particularlypreferred to use activated carbon having a particle size in the range offrom 200 to 600 μm in the present invention. If significant numbers ofparticles above or below the specified limits are present, the activityof the electrocatalytic surface will not be sufficiently high for thereduction of sulfur to occur.

Whilst not wishing to be bound by theory, it is believed that theactivated carbons used in the present invention show the unexpectedlyhigh catalytic behaviour because of the preferential chemisorption ofsulfur or oxygen species on to the surface, possibly by active sitesproduced during the activation process. The chemisorbed sulfur or oxygenspecies is then believed to act as an intermediary for a low activationenergy reduction process.

The nut shells or fruit stones which may be used to produce theactivated carbon are materials such as peach pits, almond stones, olivestones, palm nut shells, coconut shells or babassau.

The activated carbon used in the present invention also has a porevolume of from 0.45 to 1.0 preferably from 0.5 to 0.8, more preferablyfrom 0.6 to 0.7 cm³ of pore per gram of carbon. The porosity is measuredby carbon tetrachloride adsorption (for microporosity) and mercuryintrusion porosimetry (for meso and macroporosity).

The activated carbon used in the present invention also has a surfacearea in the range of from 800 to 1500 m²/g, preferably 1000 to 1100m²/g. The surface area is measured by nitrogen adsorption as describedin Porosity in Carbons, edited by John W. Patrick, Edward Arnold, 1995.

The structure which comprises the activated carbon and polymeric bindermaterial has a thickness of at least 1 mm. Preferably the structure isfrom 2 mm to 5 mm in thickness.

The polymeric binder material may be, for example, polyethylene,particularly high density polyethylene, polypropylene or polyvinylidenefluoride. Preferably the polymeric binder will be used in an amount ofup to 20% by weight, although the preferred amount will depend upon theparticular binder used. The preferred ranges are thus:

Polyethylene—5 to 15% by weight

Polypropylene—5 to 15% by weight

Polyvinylidene—10 to 20% by weight

The electrode comprises an electrode core which is in electrical contactwith the structure comprising the activated carbon/polymeric binder. Theelectrode core may be an electrically conductive carbon polymercomposite such as a high density polyethylene compounded with syntheticgraphite powder and carbon black.

The structure in electrical contact with the electrode core may be inintimate contact with the core for example by compression molding theactivated carbon polymeric binder mixture onto the surface of theelectrode or by gluing or by using heat and pressure. Alternatively, thestructure may be in electrical contact with the surface of the electrodecore via an intermediate member such as an intermediate layer of carboncloth or paper.

The electrode of the present invention may be a monopolar electrode, ora bipolar electrode in which one or both surfaces is surfaced with alayer comprising the activated carbon/binder mixture as defined above.

The electrode of the present invention may be formed by surfacing apreformed electrically conductive carbon polymer composite electrodecore with a mixture of up to 25% by weight, preferably up to 20% byweight, of a polymeric binder material and the said activated carbon. Alayer of the mixture is applied onto the surface of a preformedcomposite polymer electrode core and compression-molded onto theelectrode core to form a laminate thereon of the desired thickness.

The compression molding is preferably carried out at a temperature inthe range of from 150° to 250° C. and a pressure of from 0.5 to 5 MPa.

The compression moulding is preferably carried out at a temperature inthe range of from 150° to 250° C. and a pressure of from 0.5 to 5 MPa.

Alternatively, the activated carbon/polymeric binder may be produced astiles or sheets by appropriate thermal treatment. The tiles or sheetsare then placed into intimate contact with the surface or surfaces ofthe carbon polymer composite electrode cores, optionally by hot pressingor gluing.

Accordingly, the present invention includes within its scope a methodfor the fabrication of a carbon based electrode of the invention whichmethod comprises forming a mixture of a particulate activated carbon asdefined above with a powdered polymeric binder material in an amount ofup to 25% by weight based on the weight of the mixture, feeding the saidmixture to a mold or onto a polymeric backing sheet, subjecting themixture to a heat and pressure in order to form a sheet and eitherbonding the preformed sheet directly or indirectly to a sheet of apreformed electrically conductive carbon polymer composite electrodecore material and then cutting the bonded assembly to the desired size,or cutting the preformed sheet to the desired size to form tiles andplacing the preformed tiles directly or indirectly in electrical contactwith individual preformed electrically conductive carbon polymercomposite electrode cores.

In carrying out this method, an intermediate electrically conductivecloth or paper may be positioned between the preformed sheet or tile andthe electrical core. Intimate electrical contact may be obtained forinstance by gluing, by the application of heat and pressure or by thein-situ compression effected by the cell construction.

The present invention also includes within its scope an electrochemicalapparatus which comprises a single cell or an array of cells, each cellwith a positive chamber containing a positive electrode and anelectrolyte and a negative chamber containing a negative electrode andan electrolyte, the positive and negative chambers being separated fromone another by a cation exchange membrane and the negative electrodebeing a carbon based electrode of the present invention.

The electrochemical apparatus is preferably an apparatus for energystorage and/or power delivery. The electrolyte in the negative chamberof the electrochemical apparatus preferably contains a sulfide duringpower delivery, whilst the electrolyte in the positive chamber of theelectrochemical apparatus preferably contains bromine, iron, air oroxygen. The sulfide contained in the electrolyte in the negative chambermay be one or more of sodium, potassium, lithium or ammonium sulfide andmay preferably be present in a concentration of from 1 to 2M.

The chemical reactions which are involved in these three systems are asfollows:

Br₂+S²⁻⇄2Br⁻+S  (1)

The above reaction actually occurs in separate but dependent bromine andsulfur reactions, the bromine reaction taking place at the positiveelectrode and the sulfur reaction at the negative electrode.

2Fe³⁺+S²⁻⇄2Fe²⁺+S  (2)

Once again, this reaction actually occurs in separate but dependent ironand sulfur reactions, the iron reaction taking place at the positiveelectrode and the sulfur reaction at the negative electrode.

4H₂O+4S²⁻+2O₂⇄8OH⁻+4S  (3)

This reaction also actually occurs in separate but dependent oxygen andsulfur reactions, the oxygen reaction taking place at the positiveelectrode and the sulfur reaction at the negative electrode.

Preferably the electrodes used in the cell array as described above willbe bipolar electrodes, the negative electrode surface of which is acarbon based electrode of the invention. More preferably, however, bothsurfaces of the bipolar electrodes will be constituted by carbon basedelectrodes in accordance with the present invention.

Thus, the present invention also includes within its scope the use of acarbon based electrode as defined in a process which comprises theelectrochemical reduction of sulphur or oxygen. In particular, the usewherein the process is a process for electrochemical energy storagewhich comprises the sulfide/polysulfide redox reduction reaction.

The production of a carbon based electrode of the present invention willbe further described with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic representation of the process for the productionof a carbon based electrode.

Referring to the schematic representation of the process for theproduction of a carbon based electrode as shown in FIG. 1, theproduction of the electrode core is depicted in schematic fashion on theleft hand side of the drawing, with the production of theelectrocatalytically active tile being depicted on the right hand sideof the drawing.

In step 1 of the process for forming the electrode core a high densitypolyethylene was compounded with a synthetic graphite powder and carbonblack to form a polymeric material having a resistivity of less than 0.3ohm cm. This mixture was then extruded in step 2 of the process to forma sheet of the said polymeric material from 1 to 5 mm in thickness. Thesheet was then cut to the desired size in step 3.

In the process for forming the electro-catalytically active tile thefraction of an activated carbon from Sutcliffe Speakman Carbons Ltd.207C which passed through a 70 mesh sieve but which was retained on a 30mesh sieve was mixed (step 4) with powdered polyethylene, polypropyleneor polyvinylidene fluoride at a level not exceeding 25% by weight. Instep 5 the powder mixture is fed onto a polymer backing (PET) film bymeans of a hopper and levelled with a screen and rollers. The film ispassed under an infra-red heater array. Heat from the infra-red heatersis adsorbed by the activated carbon and conducted through the structurewhich causes the polymer to melt but does not degrade it. The composite(powder mix/backing film) is then passed through heated rollers toflatten and compress the mixture to form an electrocatalytic sheet. Thesheet is then cut to the desired size component in step 6.

An electrode is then formed in step 7 by bonding theelectrocatalytically active component (with the backing sheet removed)to the electrode core component by placing the electrocatalyticallyactive component in a compression mold tool heated to about 50° C.placing the electrode core component on top heating the electrode corecomponent to about 250° C. using an infra-red heating panel and placinga second electrocatalytically active component over the electrode corecomponent. The components are bonded together by applying a pressure of4 to 25 MPa to the composite using a piston. Both electrocatalyticallyactive sheets become bonded to the electrode core. The component is thenallowed to cool.

The present invention will be further described with reference to thefollowing Examples.

In the Examples, the following measurement techniques were used.

Overvoltage

The overvoltages were measured in a standard monopolar redox flow cellequipped with platinum reference electrodes. The results are given for a1.3M Na₂S₄ solution with balancing salts. The overvoltage is an averagevalue measured throughout the charging (reduction) period.

Porosity

The pore volume is measured by carbon tetrachloride adsorption (formicroscopy) and mercury intrusion porosimetry (meso and macroporosity)as described in Porosity in Carbons, edited by John W. Patrick, EdwardArnold, 1995.

Surface Area

Surface area is measured by the nitrogen adsorption isotherm method asdescribed in Porosity in Carbons edited by John W. Patrick, EdwardArnold, 1995.

EXAMPLE 1

An electrically conductive carbon polymer composite electrode (CPE) wasprepared from 50% by weight synthetic graphite and 50% by weight Kynar6000LD polyvinylidene fluoride powder (Elf Atochem) molded at atemperature of 210° C. and a pressure of 4.5 MPa. The CPE was surfacedwith a mixture of 14.3% by weight of a powdered polyvinylidene fluoridebinder (Kynar 6000LD—Elf Atochem) and 85.7% by weight of carbonparticles having a particle size in the range of from 212 to 600 μm (30to 70 mesh—US Standard Sieve Series) prepared by sieving an activatedcarbon sold by Sutcliffe Speakman Carbons Limited as 207C. The activatedcarbon had a surface area of 1100 m²/g and a pore volume of 0.65 cm3 pergram. The 207C/binder mixture was compression molding onto the CPE at210° C. at a pressure of 1.25 MPa to form a laminate. Various electrodeswith different surface laminate thicknesses were prepared by thistechnique.

The electrodes were each incorporated onto a monopolar cell as thenegative electrode thereof. The positive electrode was of the sameconstruction. The electrodes were separated by a cation exchangemembrane. The electrolyte in the negative compartment of the cell was 1MNa₂S₄ and the electrolyte in the positive compartment of the cell was 1Mbromine in 5M NaBr.

The results for the overvoltage for sulfur reduction are given in Table1:

Structure Overvoltage for sulfur (Thickness of activated reduction at 40MA cm⁻² carbon layer) Mv <1 mm thick 800 2.0 mm thick 57 3.0 mm thick 404.0 mm thick 43 2.0 mm thick - powder with 740 fines Ni surfacedelectrode 550

EXAMPLE 2

An activated carbon/polyvinylidene fluoride mixture was preparedaccording to Example 1. The powder mix was compression molded into anelectrocatalytic carbon tile of various thicknesses at 210° C. at apressure of 1.25 MPa. The tile was then contacted to an electricallyconducting core by positioning a sheet of electrically conducting carboncloth, paper or felt between the tile and the core by mechanicalcompression on assembly of a monopolar cell incorporating theelectrodes.

The monopolar cell was as described in Example 1 and the overvoltagesfor sulfur reduction are given in Table 2.

Structure Overvoltage for sulfur (Thickness of activated reductionscarbon layer) @ 40 mA cm⁻² 2.0 mm thick 45 mV 3.5 mm thick 52 mV 2.0 mmthick no carbon 660 mV  cloth interlayer

EXAMPLE 3

An activated carbon/polyvinylidene fluoride tile was formed according toExample 2. The tile was then bonded to the electrically conducting coreby resistive welding of the tile to form a laminate structure. For thereduction of sulfur at 40 mA cm⁻² the overvoltage was 75 mV.

EXAMPLE 4

A laminate was prepared according to Example 1 with the activated carbonpowder being bonded to both sides of the substrate sheet to form abipolar electrode. This electrode was then incorporated into a bipolarcell configuration and operated with electrolytes as detailed inExample 1. For the reduction of sulfur at 40 mA cm⁻² the overvoltage was70 mV.

What is claimed is:
 1. A carbon based electrode for the electrochemicalreduction of sulfur or oxygen, which comprises an electrode core and, inelectrical contact therewith, a structure comprising a porousparticulate activated carbon bonded with a polymeric binder material,characterised in that the structure is at least 1 mm thick, in that theparticulate activated carbon is prepared from a lignocellulosic materialand has the following properties: i) a particle size in the range offrom 200 to 850 μm; ii) a pore volume of from 0.45 to 1.0 cm³ per gram;iii) a surface area in the range of from 800 to 1500 m²/g; and in thatthe binder is used in an amount not exceeding 25% by weight based uponthe mixture of activated carbon and binder material.
 2. An electrode asclaimed in claim 1 wherein the activated carbon has a particle size inthe range of from 200 to 600 μm.
 3. An electrode as claimed in claim 1wherein the activated carbon has a pore volume of from 0.6 to 0.7 cm³per gram.
 4. An electrode as claimed in claim 1 wherein the activatedcarbon has a surface area in the range of from 1000 to 1100 m²/g.
 5. Anelectrode as claimed in claim 1 wherein said structure is in the form ofa surface layer on said electrode core and wherein said surface layer isfrom 2 to 5 mm in thickness.
 6. An electrode as claimed in any one ofthe preceding claims wherein the polymeric binder material ispolyethylene, polypropylene or polyvinylidene fluoride.
 7. An electrodeas claimed in claim 1 wherein the binder is used in an amount of up to20% by weight.
 8. An electrode as claimed in claim 7 wherein the binderis high density polyethylene which is used in an amount of from 5 to 15%by weight.
 9. An electrode as claimed in claim 7 wherein the binder ispolyvinylidene fluoride which is used in an amount of from 10 to 20% byweight.
 10. An electrode as claimed in claim 7 wherein the binder ispolypropylene which is used in an amount of from 5 to 15% by weight. 11.An electrode as claimed in claim 1 which comprises an electricallyconductive carbon polymer composite core to which the layer of activatedcarbon bonded with a polymeric binder is directly bonded.
 12. Anelectrode as claimed in claim 11 wherein the composite core compriseshigh density polyethylene compounded with synthetic graphite powder andcarbon black.
 13. An electrode as claimed in claim 1 which is a bipolarelectrode.
 14. An electrode as claimed in claim 13 wherein each surfaceof the bipolar electrode comprises a surface layer at least 1 mm thickof said particulate activated carbon bonded with a polymeric bindermaterial.
 15. A method for the fabrication of a carbon based electrodeas claimed in claim 1, which method comprises forming a mixture of saidparticulate activated carbon with a powdered polymeric binder materialin an amount of up to 25% by weight based on the weight of the mixture,applying a layer of the said mixture to the surface of a preformedelectrically conductive carbon polymer composite electrode core andcompression molding the said mixture onto the electrode core in order toform a laminate thereon of a desired thickness.
 16. A method as claimedin claim 15 wherein the compression molding is carried out at atemperature in the range of from 150° to 250° C. and a pressure of from0.5 to 5.0 MPa.
 17. A method for the fabrication of a carbon basedelectrode as claimed in claim 1, which method comprises forming amixture of said particulate activated carbon with a powdered polymericbinder material in an amount of up to 25% by weight based on the weightof the mixture, feeding the said mixture to a mold or onto a polymericbacking sheet, subjecting the mixture to heat and pressure in order toform a sheet and either bonding the preformed sheet directly orindirectly to a sheet of a preformed electrically conductive carbonpolymer composite electrode core material and then cutting the bondedassembly to a desired size, or cutting the preformed sheet to a desiredsize to form tiles and placing the preformed tiles directly orindirectly in electrical contact with individual preformed electricallyconductive carbon polymer composite electrode cores.
 18. A method forthe fabrication of a carbon based electrode, which method comprisesforming a mixture of particulate activated carbon, the particulateactivated carbon being prepared from a lignocellulosic material andhaving the following properties i) a particle size in the range of from200 to 850 μm; ii) a pore volume of from 0.45 to 1.0 cm³ per gram; iii)a surface area in the range of from 800 to 1500 m²/g; with a powderedpolymeric binder material in an amount of up to 25% by weight based onthe weight of the mixture, feeding the said mixture to a mold or onto apolymeric backing sheet, subjecting the mixture to heat and pressure inorder to form a sheet and either bonding the preformed sheet indirectlyto a sheet of a preformed electrically conductive carbon polymercomposite electrode core material and then cutting the bonded assemblyto a desired size, or cutting the preformed sheet to a desired size toform tiles and placing the preformed tiles indirectly in electricalcontact with individual preformed electrically conductive carbon polymercomposite electrode cores, wherein an intermediate electrical cloth ispositioned between the preformed sheet or tile and the electrode core.19. A method as claimed in claim 17 wherein the preformed sheet isbonded to the preformed electrically conductive carbon polymer compositeelectrode core material by the application of heat and pressure.
 20. Anelectrochemical apparatus which comprises a single cell or an array ofcells, each cell with a positive chamber containing a positive electrodeand an electrolyte and a negative chamber containing a negativeelectrode and an electrolyte, the positive and negative chambers beingseparated from one another by a cation exchange membrane and thenegative electrode being a carbon based electrode that comprises anelectrode core and, in electrical contact therewith, a structurecomprising a porous particulate activated carbon bonded with a polymericbinder material, characterised in that the structure is at least 1 mmthick, in that the particulate activated carbon is prepared from alignocellulosic material and has the following properties i) a particlesize in the range of from 200 to 850 μm; ii) a pore volume of from 0.45to 1.0 cm³ per gram; iii) a surface area in the range of from 800 to1500 m²/g; and in that the binder is used in an amount not exceeding 25%by weight based upon the mixture of activated carbon and bindermaterial.
 21. An electrochemical apparatus as claimed in claim 20 whichis an apparatus for energy storage and/or power delivery.
 22. A processfor the electrochemical reduction of sulfur or oxygen, the processcomprising the step of providing an electrochemical apparatus andelectrolytes for the electrochemical reduction of sulfur or oxygenwherein at least one electrode of the electrochemical apparatus is acarbon based electrode that comprises an electrode core and, inelectrical contact therewith, a structure comprising a porousparticulate activated carbon bonded with a polymeric binder material,characterised in that the structure is at least 1 mm thick, in that theparticulate activated carbon is prepared from a lignocellulosic materialand has the following properties i) a particle size in the range of from200 to 850 μm; ii) a pore volume of from 0.45 to 1.0 cm³ per gram; iii)a surface area in the range of from 800 to 1500 m²/g; and in that thebinder is used in an amount not exceeding 25% by weight based upon themixture of activated carbon and binder material.
 23. The process ofclaim 22 wherein the process is a process for electrochemical energystorage which comprises the sulfide/polysulfide redox reductionreaction.